Previously, pilots selected one of a number of predetermined carrier frequencies for calling a ground station which selection depended upon the distance, time and the season of the year. Thus in order for a ground station to be alert to all such calls, the receivers had to be monitored by personnel for all of the frequencies which the pilot might use. This had numerous drawbacks, namely (1) a number of people working in shifts had to be available, (2) a call occuring during traffic for the same ground station may not easily be received, (3) considerable concentration is demanded of the operating personal because they have to listen intensively for a number of hours for possible calls at all available frequencies in the noise of a full band width of receivers, and (4) the operating personnel has to pass calls on to other departments because they cannot undertake any action themselves.